International
dirtworld.com - Over 40 events listed.

mountainbikeRACER.com - A list of 90 races and events listings for Canada and the United States.

bicycletour.com - Too many to count. Mountain biking and road tours all over the world.

United States ----------------
Indiana Bicycle Coalition - Comprehensive listing

bikemonth.com - A large list of probably over 50 races and events. Most seem to be in the southern and western parts of the US.

cyclingcalendar.com - A very large listing of cycling events, with a decent amount of MTB ones

Multiple Sclerosis Society - About 100 bike tours in 48 states

a1trails.com - Annual bicycle tours and festivals in Alaska, Idaho, Oregon and Washington.

Team Big Bear - About 20 events in California

Utah.com - Over 40 listings in Utah

Canada --------------------
City of Toronto, Canada Bike Week - Hundreds of events hosted by the city and by cycling community groups.

Whistler Off Road Cycling Association - Information for British Columbia races and such.

Canadian Diabetes Association - Participate in the mountain biking event of your choice and raise money at the same time.

Europe -------------------------
Trail Break - Listings for the UK, French Alps and Peru

Adventure in Scotland - Find the latest information on all the extreme adventure sport events.

Australia -------------------
Adelaide Mountain Bike Club - A schedule of about 20 or so for Adelaide, South Australia.